1. Field of the Invention
The invention relates to a process for melting metal scrap particularly that which melts at high temperature such as steel. The invention relates particularly to a process for operating a shaft furnace to melt such scrap with fluid fuels and without the use of coke. The shaft furnace apparatus includes a furnace shaft separated from a furnace hearth located below the shaft by a cooled grate arrangement, and burners issueing into the furnace substantially at right angles to the longitudinal axis of the shaft. In the process of operating such a furnace, the combustion heat is recuperatively preheated by means of the shaft furnace waste gasses.
2. Description of the Prior Art
Processes are known for melting iron for pig iron production using cupola furnaces operated in cokeless manner. Thus, e.g. German Patent No. 22 04 042 describes a process for melting iron in a vertical shaft furnace without using coke and using a fluid fuel/air mixture which is burnt outside the furnace in burners which are positioned close to the lower part of the furnace. Iron scrap and cast iron is charged into the upper end of the furnace and melted, the superheated material dripping through a bed of loose refractory materials arranged within the shaft. The refractory materials are heated by the combustion products of the burners, before the upwardly travelling combustion products melt the metal moving towards them. Molten metal can be tapped from the bottom of the furnace.
The combustion products are introduced from all sides into the free area of the furnace from several burners, which are entirely located below the refractory material bed. The temperature of the gases in the vicinity of the bed are kept at a maximum of about 1600.degree. C. The actual furnace hearth is formed by a lower cylindrical extension of the cylindrical furnace shaft with the same diameter. Shaft and hearth are separated from one another by a water-cooled grate with the bed material arranged thereon. At the bottom of the furnace, the molten metal which has dripped through the grate collects and can be tapped off there continuously or intermittently by means of a tap hole. A similar arrangement can be used to remove the slag floating on the molten metal.
In a Hayes or Taft furnace the arrangement is such that the combustion gases from the combustion chambers meet in a central lower zone of the shaft in the so-called hearth area below the grate and, flowing upwards from there, heat the refractory bed and then, in counterflow, heat the charged material.
A further development of a cokeless cupola furnace is formed by the known Duker furnace, which includes a cylindrical shaft arrangement. A lower part of the shaft having constant cross-section is located below the water-cooled grate and forms the furnace hearth, into which oil burners issue roughly radially through the furnace wall. The molten pig iron collecting in the bottom region can be tapped off by means of a skimmer arrangement and supplied to a super heater.
A fuel oil or gas-heated shaft furnace for melting and superheating metal, specifically cast iron and copper is described in German Patent No. 38 43 678 which operates with a per charge addition of coke. The furnace has several outer burner chambers which are connected via water-cooled nozzles to the furnace interior, the burner chambers and the nozzles being located in the through-drip zone formed by the coke. Theoretically, no slag protection is required for the dripping iron in this furnace, because the protection is ensured by the reducing combustion of fuel oil and natural gas.
In connection with scrap melt-down processes, reference is also made to the shaft furnace described in German Patent No. 23 27 073, whose meltdown vessel increases in cross-section to a limited, continuous extent in the downwards direction and in which the radial burner systems are located in the immediate bottom area of the melt-down vessel where the liquid phase collects. The molten pig iron is continuously tapped off by means of a drain in the bottom. A less-than adequate separation between the solid charge material, such as e.g. iron scrap or the like, and the liquid phase is achieved due to the total omission of a separating grate arrangement in the central base of the bottom area. The presence of such a grate would insure the heat transfer between the molten material and the charge material column is reduced, or is at least kept as small as possible.
The aforementioned, cokeless-operated foundry shaft furnaces have the common feature that the furnace shaft and furnace hearth generally form a single, substantially cross-sectionally cylindrical component, i.e. a simple tube shaft. The lower portion of the tube shaft constitutes the furnace hearth with minimized surfaces for the furnace wall.
However, the known arrangement of the Flaven furnace melting unit comprises two constructional units which differ even as regards the external construction. Namely, a vertical, cross-sectionally cylindrical furnace shaft is joined to one end of a furnace hearth which is horizontal with respect thereto by means of a water-cooled grate. At the other end of the furnace hearth, a burner is situated whose flame is directed onto the tap area of the liquid phase and whose flame gases flow horizontally over the liquid bath and in guided manner to the grate arrangement and rise again from there in counterflow within the furnace shaft. The tunnel-like hearth area gives the iron bath a relatively large surface, over which the hot fuel gas can pass, without the main part of the refractory material of the hearth furnace being subject to the direct action of the burner flame. Thus, in the case of this known arrangement, the proportion of heat supplied by thermal irradiation from the hearth wall is at a minimum.
Finally, a process for melting metal in a shaft furnace using a liquid or gaseous fuel with a grate located at the lower end of the furnace for supporting the not yet molten metal introduced in the furnace is disclosed in DE-OS No. 36 10 498, U.S. Pat. No. 4,758,270, in which the oxygen-containing gas required for burning the fuel is heated prior to combustion and in which a tunnel-like furnace hearth issues horizontally into the lower part of a vertical furnace shaft. The burners are located in the end face opposite to the furnace shaft opening, so that the flame is directed in the longitudinal direction of the furnace hearth onto the inlet port in the furnace shaft, so that the flame supplies heat to the entire inner area. Consequently, the heat in the hearth walls, or the refractory material thereof, can then irradiate in the form of radiant heat onto the molten iron surface.
The known shaft melting furnaces of the described type are only suitable for pig iron production in foundries, because the relatively low temperatures attainable in the furnace hearth lead to an excessive carbon proportion in the liquid phase for steel production. Thus, foundry furnaces are only operated with a maximum of less than 40% steel scrap with respect to the charge material.
The problem of the present invention is to provide a process and an apparatus of the aforementioned type making it not only possible to use random scrap components as the charge material, but e.g. also 100% steel scrap. Simultaneously, as a function of the desired further processing, it is a problem to add molten steel in the furnace hearth while also simplifying and improving the construction of the apparatus for performing the process.